I'm not saying that the newly rich hubby has an obligation to support the kids BUT he is giving BM money (can we say income) and she is using it to bankrupt M5's hubby. This didn't happen before the windfall - it has only occurred after and because of the windfall.
Quite often, we see SAHM's being imputed their salary and new hubby paying it for their benefit to the CP for CS. Well, this isn't about the CS, it is about all the extra's that BM has signed the kids up for, even when ex says no, I can't pay, and she does it anyway.
I think since she has all this "extra" money, she should just pay for it herself for the good of the kids.
She was CO'd to also carry medical insurance on the kids, which she hasn't done. Maybe, some of what she is billing ex would have been covered by her insurance, if she had continued carrying it. Since the cost of insurance for 3-4 kids (I can't remember the number), would be at least $250/300 per month for a policy on just them, she should apply that amount of money toward any "extra" medical bills she has.